MRI safety: implants, metal and gadolinium
- Modality
The MRI magnet is always on and pulls on iron-containing metal. Every patient and object is checked before entering the room.
What it is
MRI uses no ionising radiation, but it has its own hazards, and almost all serious accidents are preventable by screening. The static magnetic field is always on, even when no scan is running: it pulls ferromagnetic objects (oxygen cylinders, scissors, wheelchairs, some older implants) towards the magnet as projectiles and can move or twist metal inside the body. The radiofrequency pulses deposit energy as heat and can burn skin where wires, leads or skin-to-skin loops concentrate current. The switching gradients make loud noise and can stimulate peripheral nerves. Active devices such as pacemakers, defibrillators, neurostimulators and cochlear implants can malfunction or heat. Each implant is labelled MR safe, MR conditional (safe only under stated conditions) or MR unsafe, and the label, not memory, decides. Gadolinium contrast adds its own small risks: allergic-like reactions, nephrogenic systemic fibrosis in severe kidney disease, and retention in the brain and bones. Access to the magnet is controlled in zones, with a safety questionnaire and a trained person screening everyone who enters.
How the image is made
- Static field: always on; projectile effect on ferromagnetic objects, and torque or movement of metal inside the body.
- Radiofrequency field: tissue heating and burns, especially with wires, ECG leads, loops of skin and some tattoos.
- Gradient fields: noise loud enough to harm hearing (ear protection for all) and tingling from nerve stimulation.
- Cryogen: the magnet is cooled with liquid helium; a quench vents it and needs a working exhaust.
- Access zones: from the public area to the scanner room, with screening before the inner zones.
What it is good for
- Screening finds what could move, heat or fail: the questionnaire, the device's labelling and, if needed, a radiograph.
- MR-conditional pacemakers and defibrillators can be scanned with cardiology reprogramming and monitoring.
- MRI without gadolinium can be used in pregnancy when it answers the question, avoiding ionising radiation.
| Item | Usual handling |
|---|---|
| Pacemaker or defibrillator | Check model and leads; MR-conditional systems are scanned with cardiology programming and monitoring |
| Older ferromagnetic aneurysm clip | Unsafe: do not scan until the clip type is confirmed |
| Metal fragment in the eye (metalworker, injury) | Screen with an orbit radiograph or CT before MRI |
| Cochlear implant, neurostimulator, drug pump | Follow the device's MR conditions; often needs the specialist |
| Joint replacement, most stents, sternal wires, dental fillings | Usually MR conditional or safe; may cause local artifact |
| Oxygen cylinder, trolley, scissors, phone | Never into the scanner room unless labelled MR safe |
Limitations and risks
- Projectile accidents can kill: only MR-safe equipment, monitors and cylinders enter the scanner room.
- Burns: remove ECG leads and patches not labelled for MRI, avoid skin-to-skin loops and pad cables away from the skin.
- A metal fragment in the orbit can move and blind the patient: ask about grinding, welding and penetrating eye injury.
- Gadolinium is avoided in pregnancy unless clearly essential, because it crosses the placenta.
- Gadolinium in severe kidney disease or acute kidney injury carries the risk of nephrogenic systemic fibrosis.
- Gadolinium is retained in brain and bone, more with linear agents; the clinical meaning is not known, so it is not used without need.
Next step
- Fill in the MRI safety questionnaire with the patient and attach implant cards or device records to the request.
- Unknown implant or device: tell the radiology department in advance; the scan waits until the model and its MR status are known.
- Pacemaker or defibrillator: arrange cardiology review for device programming before and after the scan.
- Possible metal in the eye: request an orbit radiograph or CT first.
- Contrast planned in a patient at risk of kidney disease: check eGFR; if severely reduced, discuss with the radiologist.
Test yourself
Think of the answer, then tap the question to see it.
Why is a ferromagnetic object dangerous in the MRI room even when no scan is running?
The static magnetic field is always on, so it pulls the object towards the magnet at high speed as a projectile.
What does «MR conditional» on an implant mean?
The device has been shown safe only under specific conditions (field strength, energy limits, settings) that must be met for the scan.
Which patients need an orbit radiograph before MRI?
Those who may have metal in the eye, such as welders, grinders and metalworkers or anyone with a penetrating eye injury.
How does MRI cause burns?
Radiofrequency energy heats tissue, and wires, leads or skin-to-skin loops concentrate current at small contact points.
Connections
Educational content; it does not replace a radiologist's report and is not a clinical instruction. Each study is ordered on the patient's assessment and your institution's protocols.
Sources: Magnetic Resonance Imaging (MRI) Safety and Biological Effects (StatPearls (NCBI Bookshelf), 2026).
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Representative model, for educational purposes only.